In power conversion equipment such as a 400 Hz inverter for an aircraft, power transistors which switch from non conducting to conducting states are used to control the flow of power. Increasingly, newer power conversion equipment use power transistors that are controlled by a gate terminal. These devices include field effect transistors (FETs), MOS controlled thyristors (MCTs), and insulated gate bipolar transistors (IGBTs). The voltage between this gate and a power terminal (Vg) of the power transistor must be changed to switch the conduction state. In many power electronics applications, the gate drive circuit which supplies this changing voltage must meet three basic requirements. First, it must supply dielectric isolation between a control circuit (which determines when a power transistor must conduct load current) and the transistor's terminals. Second, the drive circuit must supply a positive gate voltage (Vg) to the power transistor to ensure a conducting state, and must supply a negative gate voltage to ensure a non conducting state. Third, delays introduced by the drive circuit must be limited to a few hundred nanoseconds so that harmonic distortion is not increased in applications where precise switching angles are required, for example aircraft power generating systems.
FIG. 1 shows a gate drive circuit shown in a recent power electronics book (POWER ELECTRONICS: Converters, Applications, and Design by Mohan, Undeland, and Robbins, Wiley, New York, 1989). Although simple, this gate drive circuit suffers from an inability during startup of the power circuit to ensure a reverse gate voltage on one power transistor prior to the turn-on of another. Also, it introduces a significant turnoff delay because R2 and C2 must discharge before turnoff can occur. This delay cannot be reduced in this circuit because the time constant of R2 and C2 must be long enough to avoid inadvertent turnoff during transitions of the voltage on the transformer.
The instant invention is directed at overcoming one or more of the above problems by novel means heretofore unknown in the art.